The invention relates to a method in connection with a piston engine provided with a turbocompressor.
The invention also relates to an arrangement in connection with a piston engine provided with a turbocompressor.
At high combustion temperatures nitrogen oxides (NOx) are generated in the cylinder of a piston engine and entrained by exhaust gases to the air. Due to the detrimental environmental impacts of nitrogen oxide emissions, efforts are undertaken to prevent the generation of nitrogen oxides (primary methods), or produced nitrogen oxides are removed from the exhaust gases (secondary methods).
Adding water to the combustion process together with combustion air reduces the generation of nitrogen oxide emissions. Water lowers the combustion temperature in the cylinder, whereby less nitrogen oxides are produced. In practise, there are two alternative ways to add water to the combustion process of the piston engine. Water can be supplied either directly to the combustion chamber of the cylinder of the engine or mixed with the combustion air before it enters the cylinder.
To increase engine power, piston engines are often provided with a turbocompressor comprising a compressor, by which pressurised combustion air is supplied to the engine. Further, the turbocompressor comprises a turbine, which drives the compressor. The exhaust gases from the engine are led to the turbine, which converts the energy in the exhaust gases into driving power of the compressor.
The operation of the compressor part of the turbocompressor is restricted on one hand by the limit that corresponds to maximum capacity of the compressor and on the other hand by the so-called surge limit. Surging of the compressor is detrimental to the operation of the engine, since the pressure and flow of the combustion air supplied to the engine will decrease as soon as the compressor surges. For this reason, in order to ensure optimum operation of the compressor and the engine in changing conditions, there should be a specific safety margin between the operating point of the compressor and the surge limit. Also the operating efficiency of the compressor is at its highest, when the operating point is at a certain distance from the surge limit. Therefore, the compressor for each specific application is selected so as to operate at a required distance from the surge limit in normal operating conditions.
If a turbocharged piston engine is provided with such a system for the reduction of nitrogen oxides that water is mixed with combustion air at a point after the compressor, the water supply increases the mass flow rate of the exhaust gas passing through the turbine. Consequently, the rotational speed of the turbine is also increased, whereby the pressure of the suction air will rise after the compressor, if the engine operates at constant load. The flow of the combustion air passing through the compressor will, however, remain constant, whereby the safety margin between the operating point of the compressor and the surge limit will diminish or disappear.
As it should be possible to run the engine also in situations, where the humidification of combustion air is not in use, the compressor needs to be able to operate under two different operating conditions:
1. when the humidification of combustion air is in use, whereby the temperature of the combustion air is 70-75° C. and the mass flow rate passing through the turbine has increased to the same extent as water has been supplied,
2. when the humidification of combustion air is not in use, whereby the temperature of the combustion air is 50-55° C. and there is no extra mass flow passing through the turbine.
By using a compressor dimensioned for a lower flow rate, it should be possible to ensure a sufficient safety margin against surging, but on the other hand this involves the risk of the turbocompressor rotating at overspeed, when the system for humidification of combustion air is in operation. If the humidification system for combustion air is out of operation, the operating point of the compressor moves strongly towards the limiting value corresponding to the maximum capacity, whereby the operating efficiency of the compressor will decrease. Also, the rotational speed of the turbocompressor is in that case substantially higher compared to a situation, where a compressor according to a standard specification is used.
It is an object of the present invention is to provide a technical solution, by which the operation of the turbocompressor of a piston engine provided with humidification of combustion air can be optimised.